Land mobile radio (LMR) systems are used by municipalities, governmental entities, including police, fire, medical, and military first responders, and other organizations to provide a private, wireless communications network using mobile and/or stationary radio units to communicate with one another over a specific geographic area. An LMR system can be as simple as two subscriber units communicating between themselves and a site over preset channels, or they can be complex consisting of hundreds of subscriber units and multiple sites. Generally, an LMR system includes a plurality of sites positioned in different locations in the specific geographic area to provide the desired coverage and bandwidth. Each site may include an antenna, a site controller, and a plurality of repeaters that include a transmitter and receiver at designated or desired frequencies. In certain embodiments, the sites, such as the site controllers, may communicate with each other through a backbone data network, such as an IP data network or the Internet, to assist with communications needs between the sites, such as call-setup activities and administrative functions needed for operating an LMR system.
An LMR System may, generally, be categorized as simulcast system or a non-simulcast system. In a simulcast trunking LMR system, a channel is comprised of two or more repeaters, each having common transmit frequencies, and common receive frequencies, but located at different geographical locations. The site locations are chosen to provide a wider coverage area than is possible with a single location. In such simulcast systems, repeater failures can result in a reduction in capacity, a reduction in coverage, or both. Consider, for example in a nine channel, three site simulcast LMR system, the failure of a single repeater at one site. If the channel is allowed to be included for use in subsequent communication requests, the RF coverage will suffer, due to the loss of transmit and/or receive functionality in the geographical area normally covered by the failed repeater. If the channel is not allowed to be included for use in subsequent communication requests, the system will operate over the preferred coverage area, but with reduced capacity. In this failure scenario, it is likely that the reduced capacity result is preferred over the reduced coverage area result.
Consider an alternate failure scenario where a site loses power, causing all repeaters at that site to fail. If the same preferred action from the previous scenario were applied in this case, all channels would be excluded from subsequent communication requests, resulting in zero capacity for the system. For this failure scenario, the reduced coverage result would be preferred over the zero capacity result.
When a call is setup in an LMR system, channels, which are comprised of underlying repeaters at one or more sites, are to be effectively allocated or various problems and disadvantages may occur, such as those just described, including decreased call coverage area, decreased bandwidth or capacity, and call quality.